A number of devices are known which restrict the range of angles or positions from which a display can be viewed. U.S. Pat. No. 6,552,850 discloses a method for the display of private information on a cash dispensing machine. Light emitted by the machine's display has a fixed polarisation state. The machine and its user are surrounded by a large screen of sheet polariser which absorbs light of that polarisation state but transmits the orthogonal state. Passers-by can see the user and the machine but cannot see information displayed on the screen.
A versatile method for controlling the direction of light is a ‘louvred’ film as illustrated in FIG. 1 of the accompanying drawings. The film comprises alternating transparent and opaque layers 1 and 2 in an arrangement similar to a Venetian blind. Like a Venetian blind, it allows light to pass through it when the light is travelling in a direction 3 parallel or nearly parallel to the layers, but absorbs light travelling at large angles 4 to the plane of the layers. These layers 1, 2 may be perpendicular to the surface of the film or at some other angle. Methods for the production of such films are disclosed in USRE27617, U.S. Pat. Nos. 4,766,023 and 4,764,410.
Other methods exist for making films with similar properties to the louvred film, for example, as disclosed in U.S. Pat. Nos. 5,147,716 and 5,528,319.
U.S. Pat. No. 6,239,853 discloses a privacy device based on a different principle. The device is shown in FIG. 2 of the accompanying drawings and comprises two linear polariser sheets 5, 6 whose transmission axes 7, 8 are orthogonal. Between the sheets 5, 6 are two layers 9, 10, each of which comprises alternating stripes of isotropic material 11 and half-wave retarder 12. The layers 9, 10 are arranged so that any ray of light passing through the device in the direction perpendicular to its plane passes through exactly one half-wave retarder 12. The device is therefore substantially transparent to such rays (except for any absorption in the first polariser). If the device is placed on the front of a flat-panel display, then a viewer on the central axis will be able to read the display.
As the viewer moves to the left or right, a proportion of rays which would otherwise have reached the viewer passes through either no or two retarders 12, and so is absorbed. The device therefore partially obscures the display to off-axis viewers and provides privacy.
This device has the following disadvantages. It requires four extra layers, adding considerably to the cost and bulk of the display. It is not switchable to a public mode. As the user's head moves further to the side, past the point where all light is blocked, the display becomes visible again. In fact, light is completely blocked to viewers only at isolated points on a left-right axis.
A different method for restricting the viewing angle of a display is described in ‘Secure information display with limited viewing zone by use of multi-color visual cryptography’, H. Yamamoto et al, Optics Express v12, pp 1258-1270 (2004) and ‘Use of visual cryptography to limit viewing zone of information display’, H. Yamamoto et al, Proceedings of the 10th International Display Workshops (Fukuoka, Japan, 2003), paper VHF3-2.
The method is known as ‘visual cryptography’. The pixels of a display panel are divided into groups. In the example shown in FIG. 3 of the accompanying drawings, the display is monochrome and each group is a square of four pixels. One such square is indicated by a circle at (b). A mask shown at (c) is placed in front of the display and separated from it by a small distance. The portion of the mask in front of each pixel group contains a pattern which obscures two of the four pixels in the group. The six possible patterns are shown at (a). The choice of which pixels are obscured in each group is made randomly when the mask is designed.
An image which is to be displayed on the device (in this case, a letter ‘X’) is shown at (b). The image data sent to the display panel is shown at (d). Again, one of the six possible patterns at (a) is displayed in each group of four pixels. The choice is made so that two white pixels are visible in each group which is white in the intended image, and no white pixels are visible in each group which is black in the intended image. When the panel is viewed through the mask, a viewer in the central viewing position sees exactly two of the pixels in each group of four. For white groups in the ‘X’, these pixels are white but, for black groups in the ‘X’, these pixels are dark.
How the panel appears through the mask to a viewer in the central position is shown at (e). The ‘X’ is visible with considerable loss in image quality and brightness. When the user's head moves away from the central position, parallax causes different pixels to be exposed and the viewer sees a random dot pattern, for example as shown at (f).
This method provides privacy, but has a number of disadvantages. The brightness of the display is reduced because the mask absorbs half the light reaching it. The effective number of pixels in the display is halved. Also the random mask pattern causes the display to have a mottled appearance.
The method may be extended to colour displays, as described in the first paper mentioned above, but the disadvantages remain in this case.
Visual cryptography devices have a second application. Because the image on a display is intelligible only when the mask is present, the mask may function as a key in a cryptographic system. For example, the data to be displayed on the display panel may be sent by a transmission channel which has no security, such as a television broadcast, while the amplitude mask is possessed only by the intended receiver of the information.
There are also some applications where a user wishes to use a non-secure terminal to view information from a secure information source. For example, a user may wish to use a computer owned by a third party (for example, in a hotel or in some public place) to access bank account details. In this situation, it is useful to send an encrypted image to a display which can be read only when the user places a mask over the display. The ‘visual cryptography’ scheme may be used for this application if the user carries the mask.
The devices described above may be placed either in front of a display panel or between a transmissive display panel and its backlight to restrict the range of angles from which the display can be viewed. In other words, they make a display ‘private’. However none of them gives a method by which the privacy function can be switched off to allow viewing from a wide range of angles.
US 2002/0158967 shows how a light control film can be mounted on a display so that the light control film can be moved over the front of the display to give a private mode or mechanically retracted into a holder behind or beside the display to give a public mode. This method has the disadvantages that it contains moving parts which may fail or be damaged and that it adds bulk to the display.
A method for switching from public to private mode with no moving parts is to mount a light control film behind the display panel and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in private mode. When the diffuser is switched on, it causes light travelling at a wide range of angles to pass through the panel and the display is in public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect.
Switchable privacy devices of these types are disclosed in U.S. Pat. Nos. 5,831,698, 6,211,930 and 5,877,829. They share the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public or private mode. The display is therefore inefficient in its use of light. Since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public than in private mode unless the backlight is made brighter to compensate.
Another disadvantage relates to the power consumption of these devices. In the public mode of operation, the diffuser is switched off. This often means that voltage is applied to a switchable polymer-dispersed liquid crystal diffuser. More power is therefore consumed in the public mode than in the private mode. This is a disadvantage for mobile devices which are used for most of the time in the public mode and which have limited battery power.
A third known method for making a switchable public/private display is disclosed in U.S. Pat. No. 5,825,436. The light control device in this patent is similar in structure to the louvred film described earlier. However, each opaque element in the louvred film is replaced by a liquid crystal cell which can be electronically switched from an opaque state to a transparent state. The light control device is placed in front of or behind a display panel. When the cells are opaque, the display is in its private mode; when the cells are transparent, the display is in its public mode.
A first disadvantage of this method is in the difficulty and expense of manufacturing liquid crystal cells with an appropriate shape. A second disadvantage is that, in the private mode, a ray of light may enter at an angle such that it passes first through the transparent material and then through part of a liquid crystal cell. Such a ray will not be completely absorbed by the liquid crystal cell and this may reduce the privacy of the device.
Another method for making a switchable public/private display device is disclosed in JP2003-233074. This device uses an additional liquid crystal panel which is segmented. Different segments of the panel modify the viewing characteristics of different areas of the display in different ways, with the result that the whole display panel is fully readable only from a central position. In particular, the additional liquid crystal layer is segmented into two sets of regions. The liquid crystal is aligned differently in the two sets of regions so that, for a part of the main LCD panel viewed through regions in the first set, brightness decreases sharply as the viewer moves to the left. This decrease in brightness happens because, when light passes through a cell in the additional layer at an angle to the normal, the angle between the electric field vector and the LC director is different from that for rays passing through in the normal direction and so the polarisation changes in a different way. Similarly, when part of the main LCD panel is viewed through a region in the second set, the brightness decreases sharply as the viewer moves to the right. Changes in the display appearance as the user moves are therefore due to angle-dependence in the polarisation-modifying characteristics of the layer.
A disadvantage of this method is that the whole of the display is never obscured, so the privacy provided is not complete. Because of this limitation, the level of privacy for images or for text in large font sizes is not good.
A disadvantage of nearly all previously known privacy devices is that they are less effective under very low ambient light conditions. The only exception to this rule is the ‘visual cryptography’ method. All other previously known methods rely on preventing light from reaching unwanted viewers. Any method of doing this leaks a small amount of light, especially in areas close to the boundary between the excluded and permitted viewing zones. Under very low ambient lighting, unwanted viewers may be able to perceive this light and therefore read the display.
Autostereoscopic displays achieve a three-dimensional stereo effect by allowing the left eye of the user to see one image while the right eye sees another without the user wearing specially designed glasses. One widely-used method for electronic displays is the parallax barrier described, for example, in the book ‘Stereoscopy’ by N. A. Valyus (Focal Press, 1966), where a screen with opaque and transparent portions is placed close to the display panel, either in front or behind. When the user is in the correct position, the screen prevents light passing through some of the pixels of the display from being seen by the left eye and prevents light passing through the rest of the pixels from being seen by the right eye.
When ordinary (two-dimensional) images are shown on such a display, it is useful to switch the parallax barrier off, that is, make the entire barrier transparent. One method for achieving this is disclosed in U.S. Pat. No. 6,055,103 and EP0887666. The method uses a patterned retarder, which cannot be switched on and off, and a single switchable wave-plate.
Such a switchable parallax barrier is illustrated in FIG. 4 of the accompanying drawings and comprises an input polariser 20 whose transmission axis 21 is oriented at 0°. A patterned half wave-plate 22 comprises regions A whose optic axes are oriented at 45° to the axis 21 whereas regions B have optic axes oriented parallel to the axis 21. A uniform switchable half wave-plate 23 is switchable between the state illustrated in FIG. 4 with its optic axes 24 oriented at 22.5° to the axis 21 and another state in which the half wave-plate is effectively disabled. An output polariser 25 has its transmission axis 26 oriented at 90° to the axis 21. The switchable parallax barrier may be disposed in front of or behind a display panel to permit switching between an autostereoscopic 3D mode and a 2D or single view mode.
When the switchable wave-plate 23 is switched off, the parallax barrier is in its ‘on’ state. Light 27 passing through the lower polariser is linearly polarised with its electric field direction at 0 degrees. Those rays which pass through the regions A of the wave-plate have their plane of polarisation rotated so that the electric field direction on leaving the wave-plate 22 is 90 degrees. Rays passing through the regions B of the wave-plate 22 are unchanged in polarisation. The switchable wave-plate 23 has no effect in this state, so rays which pass through regions B are absorbed by the upper polariser 25, and rays which pass through regions A are transmitted. The structure therefore acts as a parallax barrier with regions A being transparent and regions B being opaque.
When the wave-plate is switched on, light passing through both sections A and B has its plane of polarisation rotated by the uniform half-wave plate 23 so that, when it reaches the upper polariser 25, its electric field direction forms an angle of 45 degrees with the transmission axis 26 of the polariser 25. The intensity of transmission through sections A and B is therefore the same and the parallax barrier is switched off.
Patterned wave-plates as used in this device can be made in a number of ways using polymerisable liquid crystals, including the chemicals known as ‘reactive mesogens’. Details of their fabrication are disclosed in U.S. Pat. No. 6,055,103, EP0887666, U.S. Pat. No. 6,624,863 and EP0887667. EP1047964 discloses details of refinements using multiple retarder layers to improve the variation of properties of the retarders with wavelength.